This invention relates to an improved heated fluidized bed reactor and a method for heating such reactors. More particularly, the present invention relates to an improved fluidized bed reactor used, in a preferred embodiment, for the production of polycrystalline silicon by the pyrolysis of silane containing gases.
A variety of means are well known in the art for applying the necessary heat to fluidized bed reaction zones contained within reactor vessels. A suitable heat transfer fluid and inductive or electrical resistance heaters are examples of direct means for supplying heat to the exterior of the reactor vessel. Such conventional means supply heat to the fluidized bed reaction zone by first passing heat through the reactor vessel wall. While adequate for the purposes of many applications, such means are not entirely satisfactory for other fluidized bed applications because of the particular nature of the desired reactions and the configurations of the reactors. There may be undesirable side effects that accompany such reactions using fluidized beds heated by such conventional means and configurations.
The production of polycrystalline silicone from silane containing gases in a fluidized bed reaction zone is a significant example of the limitation of conventional means for heating such fluidized beds. In this example, silicon particles are suspended in a fluidizing gas stream into which silane containing gas is injected. The process conditions are desirably maintained so that the decomposition of the silane occurs heterogeneously, i.e., the silane decomposes and deposits on the surface of the silicon particles in the fluidized bed. In this manner, the silicon particles enlarge by the deposit of silicon thereon so that sufficiently large particles of silicon product are grown to permit conventional removal of such particles from a collection zone beneath the reaction zone. The byproduct hydrogen and other gases can be separately removed as overhead gas from the reaction zone.
In polycrystalline silicon fluidized bed applications, liners have been employed on the interior of the reactor vessel walls to provide a material having the same coefficient of expansion as the silicon as well as to provide a barrier which would prevent the silicon particles from becoming contaminated by contacting the heated metal walls of the reactor vessel wall. The liners also serve to prevent silicon from becoming deposited onto the reactor vessel walls.
In the case of the pyrolysis of silane containing gas to silicon in a fluidized bed reaction zone, conventional means of heating a fluidized bed reaction zone typically employ a heat source which supplies heat uniformly to the external reaction vessel walls. The pyrolysis of silane containing gas may be achieved by capacitive heating of the fluidized bed reaction zone, as discussed in U.S. Pat. No. 4,292,344 to McHale. Other methods of heating such as uniform induction coils, electrical resistance elements and indirect gas fired heaters have also been utilized on the exterior of the reactor vessel walls and are disclosed in U.S. Pat. Nos. 3,012,861 to Ling and 3,012,862 to Bertrand, et al. These conventional means are capable of supplying the necessary heat to the external vessel walls. In certain applications, the use of uniform heating on the external walls of the reactor vessel requires the heat to pass through the vessel wall and liner resulting in a degree of conductive heat loss. Also, the use of conventional heating means often results in a temperature gradient from the bottom of the reactor to the top, i.e., a lower temperature at the bottom and a higher temperature at the top due to the lower temperature of a gas distributor and production collection chamber located below the reaction zone. When the temperature gradient is present, the lower portion of the fluidized bed is not being effectively utilized and the full benefit of the fluidized bed reaction zone is not realized.
Therefore, the need exists for the development of an improved heated fluidized bed reactor wherein it is possible to introduce more heat near the bottom than the top of the fluidized bed reactor so that full utilizaion can be made of the fluidized bed reaction zone. It is particularly desirable that such heating means be capable of supplying heat to the reaction zone without requiring the heat to pass through the reactor vessel wall.